The present invention relates to a photosensitive device read by charge transfer. It also relates to a television camera incorporating such a device.
The prior art, particularly the work by C. H. SEQUIN and M. F. TOMPSETT entitled "Charge transfer devices", pages 152 to 169 mainly describes two types of photosensitive device using charge transfers. In the first type of photosensitive charge transfer device light radiation is transmitted to charge transfer registers. There are numerous possible organizations of the registers exposed to the radiation and the charge transfer registers used for storing the charges after reading. The most frequently encountered organizations are of the "field or frame transfer type" or "the interline structure type". These devices have the disadvantage of requiring large charge transfer device surfaces, it being difficult at present to obtain high efficiency levels for the production of such large surfaces. The second type of photosensitive device is of the charge injection device-type or C.I.D. and does not suffer from the aforementioned disadvantage. These devices have a matrix of photosensitive points constituted by two MOS capacitors connected by a grid. One of the capacitors is exposed to the light radiation. The charges induced beneath this capacitor are periodically transferred to the adjacent capacitor and the charges are read by an MOS transistor. Two digital shift registers permit the addressing of each point with a view to the reading thereof. The capacitors of the same line which are exposed to the radiation are connected in parallel and addressed by one of the registers, whilst the capacitors of the same column used for reading are connected in parallel and addressed by the other register. The charge injection device have the disadvantage of requiring a re-injection of the charges into the semiconductor substrate where the capacitors are integrated during or after the reading of the charges. This re-injection requires the use of substrates which have undergone epitaxy or collecting diffusions in order to be effective. It is also difficult to control and can lead to remanence if it is incomplete. Moreover, in charge injection devices the charges are read over a high capacitance, which reduces the signal/noise ratio and said capacitance is dependent on the number of lines.
Furthermore, the article which appeared in the I.E.E.E. Journal of Solid-State Circuits, vol. SC 14, no. 3, June 1979, pp. 604 to 608 disclosed a photosensitive device having a C.I.D.-type matrix read by charge transfer (cf. in particular FIGS. 7 and 8 of the article and the comments thereon). In this device the reading capacitors are addressed by a C.C.D. (Charge Coupled Device) shift register. Each line of the matrix is read in two stages. In the first stage the potential of all the reading capacitors is fixed. There is a transverse transfer of charges into the C.C.D. register and removal of said charges by MOS transistors TG.sub.2. In the second stage the transfer of charges beneath the reading capacitors leads to the injection into the C.C.D. register of signal charges which are read in series by longitudinal transfer.
The charge transfer is of the "Bucket Brigade" type, i.e. it is carried out by means of the analog of a suturation-polarized MOS transistor.
A disadvantage of this device is that, to be effective, the transfer requires a relatively long time. However, for this device to be usable by a television camera, the first stage and also the second up to the injection of the signal charges into the C.C.D. register must take place during the line return time, which is only 12 .mu.s in the 625 line standard. During the line reading time the charges injected into the C.C.D. register are read in series and there is a re-injection of charges into the semiconductor substrate. However, an advantage of this device compared with the conventional C.I.D. devices is that the reading of the charges no longer takes place over a high capacitance and is no longer dependent on the number of lines.